There comes a point where buying 5 star packs wont help you improve your team and that's when you have to start collecting from the bottom up. It's the only way to level up too as the achievements will give you a lot of experience points. The strategy is to buy 1 star packs then 3 star then 5 star and complete collections to get elite cards which will vastly improve your team.

If you read the NHL Superstar forums it is a huge help and you can get some rewards for visiting.